Remove dead code in the cscope target

arm64 is the only currently supported architecture that has
${MACHINE_CPUARCH} set to a different value (aarch64) than ${MACHINE}
(arm64), as described in arch(7). However, there is no source directory
associated with arm64 that has a name set to ${MACHINE_CPUARCH}.

Remove the dead code that adds a directory with a name set to
${MACHINE_CPUARCH} to a list of directories indexed with cscope.
This change allows to use the cscope target on arm64.

Discussed with: jhb
Reviewed by: imp jhb
Approved by: oshogbo (mentor)
Differential Revision: https://reviews.freebsd.org/D36402